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Electric flux meaning (& how to calculate it)

Electric flux measures how much the electric field 'flows' through an area. The flow is imaginary & calculated as the product of field strength & area component perpendicular to the field. Created by Mahesh Shenoy.

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Video transcript

i'm sure you've seen these electric field lines and it's always tempting to think that you know something is actually flowing out of this positive charge or something is flowing into this negative charge even though there isn't any but in this video we're gonna take that temptation to the next level we're going to figure out how to calculate how much the electric field flows through the through a given area and that's the idea behind electric flux flux is a measure of how much electric field is flowing through something even though it's not really doing that but we imagine it to be a flow and it's a number that tells you how much the flux field is flowing and just to give you a little bit of spoiler this is what the equation of flux looks like electric flux looks like now i know this looks super complicated and everything but the goal of this video by the end of this video we would have derived this from scratch but before we begin you might be wondering why why do we even study this new concept that should always you should always keep asking that question so why are we studying this new concept of electric flux why are we imagining electric field to be a flow well we will see later on that this will help us look at coulomb's inverse square law in a completely different way it's going to blow your mind away that's what we call gauss's law it's going to come later on and that is going to help us solve certain problems which can you know which is very tedious to do just by using coulomb's law so that's where we're going towards so to eventually go towards gauss law we need to uh you know learn this vocabulary of electric flux okay imagining electric field as a flow all right so where do we start how do we start building this equation i thought maybe we can start with something that actually flows let's say water so imagine we have some water and let's say this water is flowing let's assume that the water is moving to the right with a velocity of let's take some number 10 meters per second and imagine we keep some kind of a window inside that water and let's say that window also has some area so let's say that the area of this window is i don't know maybe something like 2 meters 2 meter square sorry now the question is how much is water flowing through this window per second all right that's what i want to calculate how much water flows per second that's what we'll call as flux of water and the symbol for flux would be phi so that's what i want to calculate it'll be a great idea for you to pause and think about this this is not electricity this is nothing to do with electric fields just water and area i want you to think about it and i'll give you a small clue think about the units in which you're going to calculate this answer when i say how much water what units are you going to answer it in well how much water would be something like in liters right so we're looking at volume so we're looking at something like so many meter cubes per second so great idea to pause the video and see if you can first try and answer this yourself all right here's how i like to do it i will imagine a cube of water right behind my window and you will see in a second why i like to do that over here let's say i have a timer with me and right now the timer is zero i will start my timer and then i'll stop it after one second because i want to know how much water flows through the window in one second so let's say i start my timer and i see this cube of water flowing through the window and after one second i stop now this is the amount of water that flew through this window in one second so whatever is the volume of this cube that must be the flux does that make sense so all we have to do now is calculate the volume of this cube and again if you couldn't find the answer before again great idea to pause and see if you can do it now all right so how do we calculate volume of this cube where we do length into breadth into height so let's see if we know these things do we know what this length is do we know let me use white do we know what this length is yes we do since this is the distance traveled by water in one second remember we timed it for one second this must be 10 meters if this was 20 meters per second this would have been 20 meters so this is 10 meters so we know the length do we know the breadth and the height well we know the entire area so we can just multiply it by the area right so the area into the length would give me the cube this area would be just the area of the window and that we know is 2 meter square and so the flux of water is going to be the length 10 meters into into 2 meters square and remember this is in one second so strictly speaking i should say this is 10 meters per second into 2 meters square and that would be my flux and so what's that value that's going to be 20 meter cubed per second all right now think about it if this window was a little smaller and this cube would have been smaller so i would have smaller flux so the area matters and if the speed was less then this distance style would have been smaller making this volume smaller and again the flux would have been smaller so can you see that the flux is the product of the velocity this is the velocity and the area of that window so we can go ahead and write this in in terms of an equation now we could say the flux of water is going to be the velocity of the water times the area of that window and if this was air this would be a flux of air and this would be the velocity of air and there we have it we have calculated the flux of water but of course you say hey we don't want the flux of water we want the flux of electric field electric flux right well now what we can do is we can replace this water with electric field and so the way you think about it is you imagine the electric field is kind of the strength of the electric field is kind of like the velocity of the water so then what would be the formula for electric flux again i want you to think about this i'm pretty sure you can do this yourself i'll give you two seconds all right so the flux of the electric field right over here phi of e what's that going to be instead of velocity we'll use electric field strength we'll kind of think of this as similar to velocity but it's not nothing is moving over here but it's kind of like that so electric field strength e times the area and this is how you calculate the electric flux so if you want to calculate the flux of any field for that matter you just multiply multiplied by the field strength and the area of course over here the units would be different i'm pretty sure you can do that what will be the units of the flux over here the electric flux units would be the units of the electric field what's the unit of electric field it's force per charge remember electric field is force per unit charge right so it's going to be newton per coulomb times the area which is meter square now the units don't make much sense to me newton's new meter square per coulomb i don't know what that is supposed to be but you can kind of now understand this this number is kind of like how much the electric field is flowing through that area even though it's not really doing that but that's basically what it is so are we done well most of the hard work is done but this is not the general equation because we took a very specific case where the electric field is very uniform and the window was perpendicular to it we now have to generalize it which is just going to be little bit more steps the hard work is done okay so let's assume that the window was not perpendicular to the electric field let's assume that the window was tilted at some angle now what would be the electric flux same window same area i want you to first think about what would happen to the value of this flux would it remain the same or would it be different would it be more or less and again you can imagine this to be water if it helps you and and by the way i can i can show you a side view which is going to be a little easier to imagine all right there you go all right so pause the video and think a little bit about this what do you think will happen to the flux now through this window same area same window but tilted all right hopefully you answer you can kind of see in the drawing itself that in this in this particular window there are three electric field lines passing through but because the window has now been tilted you can kind of see that these field lines are no longer passing through it so they no longer contribute to the flux so you can kind of see that the flux has reduced because of the tilting of the window right so another question is because the flux has changed now comes the question how do we calculate the electric flux now you can't just multiply electric field into the area this would be true if it was perpendicular but what now how do we calculate it well here's how i like to look at it let's imagine let's get rid of that window and let's imagine that you're looking from this side in the direction in which the electric field is sort of flowing that's what we're imagining now as far as you are concerned the top of the window is somewhere over here at this level and the bottom of the window from your angle is somewhere over here so as far as you are concerned for you the window through which the electric field is flowing is just this this is the effective area we could say through which the electric field is flowing and so now to calculate the electric flux you have to multiply with not this area but this area the area that the component of the area that's perpendicular to the electric field and we can go ahead and call that area as a perpendicular if you call that then all you have to do is you know calculate what the a perpendicular is and multiply it by that so our simple modification to this equation is you calculate you multiply it with the a perpendicular and if you're wondering how do i calculate this value this effective area it can be done with trigonometry if you knew this angle then you can use trigonometry and figure this out but don't worry we'll do that some in some other video not over here and now hopefully you'll agree as you tilt it more and more as the angle becomes more and more the effective area reduces the a perpendicular becomes smaller and smaller and eventually when the window becomes completely parallel to the electric field the effective area goes to zero and that's when we say the flux is zero and hopefully that makes sense now nothing is flowing through this window because it is parallel to the floor parallel to the electric field all right so the angle matters as well so is this the most general equation not really there comes a last generalization because over here we assume the electric field was constant or uniform over the entire window but what if that's not the case what if we have a non-uniform electric field and let's say the area is also all you know curvy and everything now how do you calculate the flux the electric field is changing everywhere so what value will you substitute and how do you calculate a perpendicular the angle is changing everywhere now what do you do again one last time i want you to pause the video and think a little bit about how would you calculate it well now what we could do is we can take that area and we can divide it into tiny tiny little pieces very very very very tiny pieces and the idea is if the piece is small enough you can assume each of these piece to be very flat and you can assume the electric field over that piece is pretty much uniform you make it small enough and you can assume that the electric field over there is uniform and then you can calculate what the flux is over there and do the same thing and add up and so if you were to write it mathematically the way we do that is let's assume a tiny piece over here let's zoom into that section over there so let's take the tiny piece and it's area we're going to call it as da in the speed of calculus because this is an infinitesimally small piece that's how we like to think about it it has an infinitesimally small area we would say d a and let's say the electric field at that point at that point is somewhat this way which has an electric field strength is e then how do we calculate flux over here well the same formula the flux would be e times the perpendicular component of that area so the tiny tiny flux over there we'll call d phi the tiny flux over here is going to be the electric field at that point times the perpendicular tiny area over there and then you calculate that same thing everywhere and you add up and when you add up you're doing an infinite summation because you're taking these very tiny pieces uh in a limit that the the area is about to go to zero and it's the calculus now so when you add up we use now the term integral so the total flux the total flux which we're going to call this phi now the total flux now becomes the integral of e times d a and so that's how you calculate flux in general this is the most general equation now to calculate flux of course we can also write this in the vector form which we'll see in another video but yes this is where we stop so to sum it up electric flux is a number which tells you how much electric field is quote unquote flowing through an area and the way to calculate that is we multiply the field strength with the perpendicular component of the area and of course if the field is changing everywhere and the area is all crooked and everything then you calculate the flux over a tiny surface and then you integrate it over the entire surface and remember this is not just the flux for electric field tomorrow if you're learning about magnetic flux just replace e with b this can be a flux of any field that you want